View source for Counter (digital)

===Pulse width measurement===

[[Pulse width]] measurement is a common counter application that is used in a wide variety of equipment, including [[radar]] and [[sonar]], [[industrial automation]], and [[medical imaging]] systems. A typical circuit is shown below, which uses a binary up-counter to measure the widths of asynchronous (with respect to the counter clock) positive pulses.

[[File:Pulse width measurement circuit.jpg|500px|Schematic diagram of a pulse measurement circuit based on a binary up-counter]]

The measured signal is first synchronized to the counter's clock domain, thus producing the synchronized input signal ''Clock gate''. This is done to prevent measurement errors due to [[metastability (electronics)|metastability]] or violations of minimum [[Flip-flop (electronics)#Timing considerations|setup or hold times]] in the counter and edge detector flip-flops.

When a ''Clock gate'' pulse begins, the rising edge detector strobes ''Start'' to zero the count, and the counter then proceeds to count clock pulses while ''Clock gate'' remains active. When the pulse ends, the counter stops counting and the accumulated count indicates the measured pulse width in units of clock periods. The pulse width is <math display="inline">Count / f_{clock}</math> seconds, where <math>f_{clock}</math> is the clock frequency in Hertz.

A falling edge detector strobes ''End'' to indicate end of measurement, which can be used to signal external circuitry or transfer the count to external storage, or both, before the next measurement begins.